1. Field of the Invention
The present invention relates to a thermosensitive recording material, in particular, to a thermosensitive recording material having a surface glossiness of 50% or more {GS (75.degree.)} in accordance with Japanese Industrial Standards (JIS)-P-8142 and excellent matching properties to a thermal head.
2. Discussion of Background
A thermosensitive recording material, which is one of recording materials, is constructed in such a manner that a thermosensitive coloring layer capable of forming color images under application of heat is formed on a substrate such as a sheet of paper, synthetic paper or a resin film. The thermosensitive recording material is used in a thermal printer equipped with a thermal head, which supplies the thermal energy to the above thermosensitive recording material to form images thereon. This image recording method by use of the thermosensitive recording material has many advantages over other conventional recording methods. For example, the image recording by the thermosensitive recording material does not necessitate development and image-fixing processes, the image recording can be performed by a relatively simple mechanism in a short time, and the manufacturing cost of the thermosensitive recording material itself is low. Therefore those thermosensitive recording materials are utilized not only in copying machines, but also in facsimile apparatus, ticket vending apparatus and label recorders.
Conventional thermosensitive recording materials, however, are not necessarily perfect in every respect. More specifically, a thermofusible material contained in the thermosensitive recording material is melted and adheres to a thermal head in the course of image-recording, which induces a sticking phenomenon, causes the thermal head to wear away, and impairs the quality of recorded images.
In particular, when the thermosensitive recording material is used as a ticket or label paper, the reliability of recorded images is not stable and the resistance to water, plasticizers and oils is not sufficient. In addition to the above, when the thermosensitive recording material is practically used as a ticket or label paper, its printabilities, such as ink receptivity and ink adhesion, are inferior to those of plain paper.
The above-mentioned problems have been solved to some degree, but not completely, by providing a protective layer on the thermosensitive coloring layer in the thermosensitive recording material. The protective layer generally comprises a resin, a pigment, a lubricant or a surface active agent.
In the formation of the protective layer in the thermosensitive recording material, finely-divided particles of an inorganic pigment such as silica, calcium carbonate, aluminum hydroxide, titanium oxide, barium sulfate, zinc hydroxide, clay, talc, surface-treated calcium and surface-treated silica, each having an average particle diameter of 0.5 .mu.m to 4 .mu.m; and finely-divided particles of an organic pigment such as urea - formaldehyde resin and polyethylene resin, each having an average particle diameter of about 0.5 .mu.m, are conventionally employed.
As a water-soluble binder for use in the protective layer, water-soluble polymers such as polyvinyl alcohol, modified polyvinyl alcohol, cellulose derivatives, sodium polyacrylate, polyvinyl pyrrolidone, acrylamide - acrylic acid ester copolymer, acrylamide - acrylic acid ester - methacrylic acid terpolymer, alkali salts of styrene - maleic anhydride copolymer, alkali salts of isobutylene - maleic anhydride copolymer, polyacrylamide, gelatin and casein; emulsions such as polyvinyl acetate, polyurethane, polyacrylic acid, polyacrylic acid ester, vinyl chloride - vinyl acetate copolymer, polybutylmethacrylate and ethylene - vinyl acetate copolymer; and latexes such as styrene - butadiene copolymer and styrene - butadiene - acrylic acid derivatives copolymer are conventionally employed
Metallic salts of aliphatic acids such as zinc stearate, aluminum stearate, calcium stearate, zinc palmitate and zinc behenate are used as a lubricant for use in the protective layer.
When the protective layer of the thermosensitive recording material comprises the above-mentioned binder agent, pigment and lubricant, the matching properties of the recording material to a thermal head (hereinafter referred to as the thermal-head matching properties), reliability of recorded images and printability can be quite improved, but not perfectly.
However, the surface glossiness of the conventional thermosensitive recording materials attains 1 to 20% {GS (75.degree.)} in accordance with JIS-P-8142, even when the mixing ratio of the above components is controlled with the utmost care so as to maintain the necessary thermal-head matching properties, reliability of recorded images and printability, or the conditions of supercalendering for surface-treatment are improved.
Recently, there is a tendency to impart a high-grade impression to thermosensitive recording products, such as a label or ticket sheet using the thermosensitive recording material. There is a demand for a thermosensitive recording material having high surface glossiness. However, in the above-mentioned conventional thermosensitive recording material comprising a protective layer, it is difficult to obtain a sufficient glossiness because the protective layer contains pigment particles.
In Japanese Laid-Open Patent Application No. 61-262177, there is proposed a thermosensitive recording material in which a protective layer comprising finely-divided particles of calcium carbonate having an average particle diameter of 6 .mu.m or less is provided on a thermosensitive coloring layer. The thermosensitive recording material proposed in this application, however, is not directed to the improvement of the surface glossiness of the recording material, but to the mere improvement of the thermal-head matching properties thereof.
To obtain sufficient surface glossiness of the thermosensitive recording material, there is proposed a method of further coating an ultraviolet-curing resin on the protective layer. However, this method has another product quality problem of the occurrence of cracking or crazing in the surface thereof, and the manufacturing cost is high.